Ikechukwu V. Agomoh

Ammonia loss following nitrogen fertilization can degrade air quality and impact human health, whereas nitrous oxide (N ₂ O) can contribute to global warming and climate change. Mitigation practices that target only one N‐loss pathway can lead to pollution swamping; hence, practices targeting both N‐losses are required. A 3‐year field study examined fertilizer N‐placement (broadcast urea, single‐slot injection of urea ammonium nitrate [UAN], double‐slot UAN injection) and N‐metabolization inhibitors (with/without urease and nitrification inhibitors) impacts on NH ₃ and N ₂ O losses and corn yields. Ammonia volatilization was reduced ( p < 0.05) by 26% with single‐slot UAN injection (10.6 kg N ha ⁻¹ ) and by 63% with double‐slot UAN injection (5.32 kg N ha ⁻¹ ) compared to broadcast urea (14.3 kg N ha ⁻¹ ). Dual urease and nitrification inhibitors reduced NH ₃ volatilization (0.84–3.86 kg N ha ⁻¹ ) by 57%–92% compared to no inhibitors (5.32–14.3 kg N ha ⁻¹ ). When no inhibitors were applied, N ₂ O emissions from slot injection (6.43–7.62 kg N ha ⁻¹ ) were 2.6–3.1 times greater than from broadcast urea (2.43 kg N ha ⁻¹ ). Dual inhibitors reduced N ₂ O emissions by 43% from 6.43 to 3.66 kg N ha ⁻¹ with double‐slot injection. Double‐slot UAN injection increased corn grain yields (9.73 t ha ⁻¹ ) by 12%–13% compared to single‐slot UAN injection (8.71 t ha ⁻¹ ) and broadcast urea (8.6 t ha ⁻¹ ). Double‐slot UAN injection effectively decreased NH ₃ losses and increased corn grain yields, but dual N inhibitors were required to also reduce N ₂ O. Hence, combined productivity and environmental benefits were accrued only when fertilizer containing urease and nitrification inhibitors was combined with double‐slot injection.

Corn (Zea mays L.) stover is an increasingly important feedstock for biofuel and bioproduct production; however, excessive harvest of corn stover from agricultural fields may affect corn seedling emergence and subsequent plant population which may reduce corn growth and yield. Field experiments were conducted over four consecutive growing seasons in southwestern Ontario to determine under continuous corn cropping system the effects of no‐tillage (NT), conventional tillage (CT), and five rates of stover harvest (0, 25, 50, 75, and 100 wt. %) on corn seedling emergence in a Harrow sandy loam and in a Brookston clay loam. Time to achieve 50% seedling emergence (t_0.5), emergence rate constant (k_G), and final seedling population (M_G) were adequately simulated using the Gompertz function. Irrespective of stover removal rate and soil type, t_0.5 was 2 d longer, and k_G was 32% lower, under NT compared to CT. In Harrow sandy loam, stover removal from NT reduced t_0.5 in 1 out of 4 yr. The 100% removal reduced t_0.5 relative to 0, 25, 50, and 75% removal by 1.1–2.2 d in 2015. Averaged across both tillages and all removal rates, k_G was greater in Harrow sandy loam than in Brookston clay loam. Stover removal rates had no effect on M_G but averaged over all removal rates and soil types, M_G was 9% lower for NT compared to CT in 2016. No‐tillage practice under corn production can impede seedlings emergence early in the growing season and stover removal from the field can help mitigate against this effect.

Soybean ( Glycine max L.) is known to contribute to soil N reserves when grown in rotation with other high‐value crops such as corn ( Zea mays L.) and winter wheat ( Triticum aestivum L.). However, continuous soybean and “short” soybean rotations (e.g., corn–soybean, wheat–soybean) may cause declining soybean yields and degrading soil health over time. In this long‐term field study, we determined crop rotation effects on soybean yield and soil health indicators for a cool, humid, clay loam soil in southwestern Ontario. The study used nine soybean rotations, which included continuous soybean (S), corn–soybean (C–S), corn–soybean–soybean (C–S–S), and six rotations where winter wheat (WW) was grown with red clover ( Trifolium pratense L.) (+RC) and without underseeded red clover (i.e., winter wheat–soybean [WW–S, WW+RC–S], corn–soybean–winter wheat [C–S–WW, C–S–WW+RC], and winter wheat–soybean–soybean [WW–S–S, WW+RC–S–S]). Ten soil health indicators during the first and second soybean phase of each rotation were measured in 2018, whereas soybean yield was measured from 2002 to 2018. Soybean yields in 2‐ and 3‐yr rotations were 39–44% and 48–52% greater, respectively, relative to continuous soybean excluding the rotations with 2 of 3 yr of soybean (C–S–S, WW–S–S, WW+RC–S–S), which were only 22–35% greater than continuous soybean. Partial least squares regression indicated that inorganic N, particulate organic matter N, particulate organic matter C, potentially mineralizable N, total C, soil respiration rate, and water extractable organic soil C were the most important soil health indicators, explaining 34% of the total variation in soybean yields. It was concluded that soybean grown in 3‐yr rotations with corn and winter wheat produced the largest soybean yields and the greatest positive impacts on soil health indicators likely owing to cereal crops enhancing C inputs into soil.

The soil microbial community (SMC) and soil organic matter (SOM) are inherently related and are sensitive to land‐use changes. Microorganisms regulate essential soil functions that are key to SOM dynamics, whereas SOM dynamics define the SMC. To expand our understanding of soil health, we evaluated biological and SOM indicators in long‐term (18‐yr) continuous silage corn (Zea mays L.), continuous soybean [Glycine max (L.) Merr.], and perennial grass ecosystems in Ontario, Canada. The SMC was evaluated via ester‐linked fatty acid methyl ester (EL‐FAME) and amplicon sequencing. Soil organic matter was evaluated via a new combined enzyme assay that provides a single biogeochemical cycling value for C, N, P, and S cycling activity (CNPS), as well as loss‐on‐ignition, permanganate oxidizable C (POXC), and total C and N. Overall, soil health indicators followed the trend of grasses > corn > soybean. Grass systems had up to 8.1 times more arbuscular mycorrhizal fungi, increased fungal/bacteria ratios (via EL‐FAME), and higher microbial diversity (via sequencing). The POXC was highly variable within treatments and did not significantly differ between systems. The novel CNPS activity assay, however, was highly sensitive to management (up to 2.2 and 3.2 times higher under grasses than corn and soybean, respectively) and was positively correlated (ρ > .92) to SOM, total C, and total N. Following the “more is better” model, where higher values of the measured parameters indicate a healthier soil, our study showed decreased soil health under monocultures, especially soybean, and highlights the need to implement sustainable agriculture practices that maintain soil health.

Demand for food is rising and the ability of any particular soil to support sustainable food production is dependent upon a variety of soil biochemical, chemical and physical soil parameters. However, the challenge is that the impacts of a new management practice on soil properties may take years to assess. In this field study, we investigated the effects of long‐term cropping rotation treatments (established in 2001) on soil health indicators from soils under monoculture, 2‐yr and 3‐yr crop rotations with and without a cover crop. In particular, we compared soil heath indicators under eight cropping sequences including continuous winter wheat ( Triticum aestivum L.) (WW), soybean ( Glycine max L.)–winter wheat (S–WW), corn ( Zea mays L.)–soybean–winter wheat (C–S–WW), and winter wheat–soybean–soybean (WW–S–S) all with and without red clover ( Trifolium pretense L.) (RC) in the wheat phase of the rotation. We measured ten soil health indicators collected from the wheat phase of the rotation. Crop rotation had greater effects on soil health indicators than cover crop. After 17 years, crop yields were 23–28% greater for the 2‐yr and 3‐yr rotations than monoculture WW in the presence of red clover. In the absence of red clover, yields were 32–39% greater for the 2‐yr and 3‐yr rotations than monoculture WW. However, the soil health indicators were significantly greater for monoculture WW than the 2‐yr and 3‐yr rotations. These results suggest WW enhanced soil health while crop rotations with soybean negatively impacted most biochemical soil health parameters.